A heatwave occurs when a region experiences an unusually prolonged period of temperatures that are exceptionally high relative to the local climate and season. There is no single worldwide temperature threshold that defines a heatwave because what constitutes extreme heat depends on the normal climate of the location, the time of year and sometimes the duration and minimum nighttime temperatures. A temperature that is extraordinary in a cool northern climate may be common during summer in a hot desert climate.
Heatwaves therefore represent climatic extremes relative to local conditions rather than simply absolute high temperatures. They are produced by atmospheric circulation patterns that cause heat to accumulate or persist over a region. Persistent high pressure is one of the most important mechanisms because sinking air suppresses cloud formation and precipitation while clear skies permit strong solar heating of the land surface. When the same air mass remains over an area for several days, heat can accumulate progressively. Soil moisture, vegetation, humidity, wind, ocean temperatures, urbanisation and atmospheric circulation all influence the intensity and duration of the resulting event.
Persistent high pressure, often called an atmospheric blocking pattern when it substantially disrupts the normal movement of weather systems, is a major cause of many heatwaves. In a high pressure system, air tends to descend through the atmosphere. As descending air moves into higher pressure near the surface, it is compressed and warms. This process is called adiabatic compression or subsidence warming.
High pressure also tends to produce relatively clear skies, allowing strong incoming solar radiation to heat the surface during the day. If the pressure pattern remains stationary or moves slowly, the region can remain under these conditions for many days. In some situations, a strong upper level ridge extends over a large area and helps maintain persistent heat. Atmospheric blocking can prevent cooler maritime air or travelling low pressure systems from reaching the affected region. The combination of subsidence, sunlight, suppressed rainfall and slow weather system movement can therefore create a self reinforcing period of exceptional warmth.
Heatwaves are also influenced by the origin and movement of air masses. A region can experience extreme heat when winds transport very warm air from a hotter continental or subtropical area. This process is sometimes called warm air advection. For example, a particular weather pattern may draw hot air northwards from lower latitudes, where intense solar heating has already warmed the atmosphere and land surface.
If the transported air then remains beneath a persistent high pressure ridge, local heating can intensify the event further. Air arriving from warm ocean regions can also contribute to high temperatures, although humid air behaves differently from dry continental air. The movement of air masses means that heatwaves are not always caused solely by local heating. Large scale circulation patterns involving the jet stream, pressure systems, atmospheric waves and interactions between continents and oceans can determine where exceptionally warm air develops and where it is transported.
Land surface conditions strongly affect the severity of heatwaves. When soils are moist, a significant portion of incoming solar energy is used to evaporate water through evaporation and plant transpiration, a combined process known as evapotranspiration. This transfers energy into latent heat rather than converting as much of it directly into sensible heating of the land and near surface air. During drought, however, soil moisture can become depleted and vegetation may close its stomata or become stressed, reducing evapotranspiration.
More available energy can then contribute directly to surface warming. This creates an important land atmosphere feedback in which dry conditions can intensify heat and heat can worsen soil moisture loss. The effect is particularly important over continental interiors and regions already experiencing water scarcity. Vegetation type, soil characteristics, irrigation, land use and recent rainfall can therefore influence how strongly a particular region responds to the same large scale atmospheric circulation pattern.
Nights are an important component of heatwave severity because human and ecological systems depend on nighttime cooling. Under clear skies, the ground can lose heat through outgoing longwave radiation, but the extent of cooling depends on humidity, cloud cover, wind, soil moisture and surface characteristics. Heatwaves can produce especially dangerous conditions when minimum temperatures remain unusually high for several consecutive nights. Warm nights reduce the opportunity for people, buildings and ecosystems to recover from daytime heat.
High humidity can also increase heat stress because evaporation of sweat becomes less effective as the air approaches saturation. This is why measures such as wet bulb temperature and heat index can provide additional information beyond ordinary air temperature. Urban areas can experience stronger nighttime heat because buildings, roads and other surfaces absorb and store solar energy during the day and release it slowly after sunset. The urban heat island effect can therefore amplify heatwave exposure in densely developed areas.
Climate change is altering the background conditions in which heatwaves occur. Human caused increases in greenhouse gas concentrations have warmed the global climate, shifting the distribution of temperatures towards higher values. Consequently, extreme hot events can become more frequent, more intense and longer lasting in many regions even though individual heatwaves remain strongly influenced by natural atmospheric variability. Climate change does not mean that every heatwave is caused entirely by human activities.
Individual events arise from particular weather patterns, while long term warming changes the baseline on which those weather patterns operate. Scientists use climate attribution methods to estimate how the probability or magnitude of a particular extreme event has changed because of human caused warming. The results vary among events and regions, but the physical expectation is straightforward: when the average climate is warmer, a fixed circulation pattern can produce higher absolute temperatures and more frequent exceedance of historical extreme thresholds.
Heatwaves can have major consequences for human health, agriculture, infrastructure, ecosystems and energy systems. Extreme heat increases the risk of heat exhaustion and heatstroke and can worsen existing cardiovascular, respiratory and other health problems. High temperatures can increase electricity demand because of air conditioning, potentially placing stress on power systems. Crops can suffer when temperatures exceed physiological thresholds, particularly when heat coincides with drought and inadequate irrigation. Livestock can experience reduced productivity or mortality under severe heat stress.
Roads, railway infrastructure, electrical equipment and other systems can also be affected by thermal expansion and reduced operating efficiency. Ecosystems may experience changes in plant productivity, animal behaviour, wildfire risk and water availability. The effects depend not only on temperature but also on duration, humidity, nighttime recovery, population vulnerability, access to cooling, housing quality, public health preparedness and the timing of the event.
Heatwaves are forecast using observations and numerical weather prediction models that simulate atmospheric dynamics, radiation, moisture, land surface processes and ocean interactions. Meteorologists monitor pressure patterns, upper atmospheric ridges, soil moisture, sea surface temperatures and forecast air masses to identify conditions favourable for prolonged heat. Warning systems often use locally defined thresholds based on health impacts rather than a universal temperature value.
Adaptation measures include heat health warning systems, public cooling facilities, urban tree cover, reflective surfaces, building insulation, ventilation strategies, changes to work schedules and protection for vulnerable people. Longer term planning also considers how infrastructure and urban design can cope with a warmer climate. A key misconception is that a heatwave is simply any period of hot weather. Scientifically, its significance comes from unusual persistence or intensity relative to local climate conditions, and its causes involve interactions between large scale atmospheric circulation, land surface processes, moisture, ocean conditions and the changing climate background.
